Concession roads
In Upper and Lower Canada, concession roads were laid out by the colonial government through undeveloped Crown land to provide access to rows of newly surveyed lots intended for farming by new settlers. The land that comprised a row of lots that spanned the entire length of a new township was "conceded" by the Crown for this purpose (hence, a "concession of land"). Title to an unoccupied lot was awarded to an applicant in exchange for raising a house, performing roadwork and land clearance, and monetary payment. Concession roads and cross-cutting sidelines or side roads were laid out in an orthogonal (rectangular or square) grid plan, often aligned so that concession roads ran (approximately) parallel to the north shore of Lake Ontario, or to the southern boundary line of a county. In a common square grid layout known as a 1,000 Acre Sectional System, adjacent parallel roads were 100 chains or 1.25 miles (2.0 km) apart, and arranged as 10 100-acre lots each 20 chains by 50 chains so that two consecutive concession roads and two consecutive side roads enclosed a square of 1,000 acres (4 km2). Other plans used during colonial surveying used different layouts and lot sizes of 100, 150, 160, 200 or 320 acres.
Concession roads were typically numbered consecutively. The first concession road in a township, though, was often called the baseline (from the surveying term), and roads so named remain in many municipalities, including Ottawa, Clarington and London; the first concession road was also frequently called the front, or broken front if it was on a lake shore. In some areas it's referred to as a townline, particularly if it is a boundary line between two townships, e.g., Adjala-Tecumseth Townline.
In some townships, such as those in Bruce County, each side of each successive concession road comprised a separate numbered concession. Thus, the south side of a road might be Concession 2 and the north side Concession 3. In this system, for the purposes of road signing only even (or, sometimes, odd) numbers were used, so that concession roads were successively numbered, e.g., 2, 4, 6, etc. This simplified the address numbering of farm lots, especially along township boundary roads where opposite sides of the same road were in different townships. Where even numbers were used, the numbers of odd-numbered concessions would appear only in property records (e.g., Lot 18, Concession 11, Brant Township, which would be on the north side of Concession Road 10).
...
Wikipedia